1. Field of the Invention
The present invention relates generally to a bandpass filter including at least two surface acoustic wave (SAW) resonator filters coupled in parallel and utilizing longitudinal resonance modes, and more specifically to such a bandpass filter which features a high degree of design freedom.
2. Description of Related Art
It is well known in the art that SAW resonator filters have been widely used in telecomunication equipment wherein a narrow passband and low insertion loss are required.
Before turning to the present invention it is deemed preferable to discuss, with reference to FIGS. 1 and 2, a bandpass filter which appears important for an understanding of the present invention.
FIG. 1 schematically illustrates a conventional bandpass filter (depicted by numeral 4). This filter 4 consists of two saw resonator bandpass filters 6 and 8 which are electrically coupled in parallel. The SAW resonator filters 6 and 8, each using longitudinal resonance modes, are deposited on a piezoelectric substrate 10 in a wave propagation (viz., longitudinal) direction.
The SAW resonator filters 6 and 8 are configured in exactly the same manner except for the electrode widths thereof so that the pass band center frequencies of the two filters are different.
As shown in FIG. 1, the SAW resonator filter 6 includes two interdigital transducers (IDTs) 12a and 12b, and two grating reflectors 14a and 14b. On the other hand, the other SAW resonator filter 8 includes two IDTs 12a' and 12b', and two grating reflectors 14a' and 14b'.
An incoming signal is applied to the IDTs 12a and 12a' via an input terminal 16, while a filtered signal is obtained from an output terminal 18.
It should be noted that the filter characteristics of the two filters 6 and 8 are exactly identical with each other except for their passband center frequencies. As is well known in the art, the center frequencies are made different if the electrodes of the filter 6 are different from those of filter 8.
The frequency responses of the two SAW bandpass filters 6 and 8 are combined to obtain a passband which is wider than each of the SAW filters 6 and 8.
In FIG. 1, notations D1-D3 and D1'-D3' depict the distances between the elements of the SAW filters 6 and 8, as clearly shown in FIG. 1.
FIG. 2 is a sketch schematically showing frequency-loss characteristics of the bandpass filter 4 (denoted by a solid line 20). These filter characteristics can be obtained by combining those of the two filters 6 and 8 (denoted respectively by dashed lines 22 and 24).
It is understood that the low and high sides of the resultant bandpass (denoted by solid line 20) are determined by the corresponding low and high sides of the bandpass exhibited by the filters 6 and 8.
Accordingly, by way of example, if it is required that the resultant passband should have a sharp slope at the high side while maintaining at the relatively gradual slope the low side, it is not possible to meet such requirements with the above mentioned conventional circuit.
It is therefore highly desirable to expand or increase a degree of design freedom of such a type of composite bandpass filter.